Refractory shotcreting has become a major process for the repair and maintenance of refractory linings in steel, non-ferrous metal, chemical, mineral and ceramic processing plants.
In recent years, cement-free refractory castables suitable for use in high temperature applications and having excellent resistance to corrosion by steel making slags have been developed. These refractories typically utilize a hydratable alumina binder in place of calcium aluminate cement. Because of the suitability of these refractory castables for high-temperature applications in the steel-making industry, it is highly desirable to be able to apply this material by a wet-mix shotcreting process.
Shotcreting is usually classified according to the process used, i.e., wet-mix or dry-mix spraying. A wet-mix refractory shotcrete process generally consists of thoroughly mixing a refractory material and water to produce a pumpable mixture, then introducing the mixture into a delivery hose and pumping the mixture to a dispensing (i.e., spraying) nozzle. A dry-mix refractory shotcrete process generally consists of conveying a dry refractory mixture through the delivery hose by compressed air, then introducing water under pressure at the nozzle prior to application of the refractory to the target surface. In both processes, a set modifying admixture may typically be added to the refractory castable at the nozzle to initiate and accelerate the thickening and setting of the refractory castable so that it will not slump or sag when applied to vertical or overhead surfaces. Since the mixture must be fluid for pumping in the wet-mix process, such admixtures are more critical in the wet-mix compared to the dry-mix process.
Heretofore, refractory shotcreting has found limited application in high temperature and severe corrosive applications, such as for example, in the steel making industry where the refractory material may be exposed to temperatures in the range of 2700.degree. F. to 2900.degree. F. and come in contact with corrosive slag materials. A main reason that shotcreting of refractories is limited in such applications is due to the materials added to the refractory compositions in order to facilitate their application by a wet-mix or a dry-mix shotcreting process. In both processes, a refractory castable mix is used as the base refractory composition. Most dry-process gun mixes are based on castable mixes that are modified to have finer (i.e., smaller) aggregate (than the castables) to reduce "rebound" when the refractory is applied to a surface. In addition, additives, usually clay, are added to the refractory to enhance the sticking (i.e., non-slumping) characteristic of the material when applied to vertical walls. These modifications make dry gunning mixes less applicable to severe high temperature, corrosive applications as compared to castables of similar base composition because the clay adds considerable silica to the refractory. The addition of the silica to very pure alumina systems may have a detrimental effect particularly on the hot modulus of rupture (MOR) of the material, in high temperature shrinkage and resistance to corrosion by steel making slags. In this respect, such gunning mixes may have lower strength at steel making temperatures in the range of 2700.degree. F. to 3000.degree. F. and exhibit a permanent linear change (i.e., shrinkage) at certain temperatures, i.e., above 2700.degree. F. In addition, such gun mixes typically have high porosity, which is related to the clay addition, altered grain sizing and incomplete mixing inherent in a dry gunning process. As a result, such gun mixes are less than desirable in repairing steel ladle linings or other surfaces where such linings would be exposed to extremely high temperatures, i.e., above 2700.degree. F., and to highly corrosive slags.
A wet-mix shotcreting process overcomes the less than desirable mixing inherent in a dry-gunning process and does not generally require the addition of clay to the base refractory castable composition used. Accordingly, a refractory castable suitable for high temperatures and corrosive use may find advantageous application in a wet-mix process, but only if a suitable "accelerator" can be found for use therewith. In this respect, to facilitate the wet-mix process, a set modifying admixture, conventionally referred to as an "accelerator" must be added to the wet-mix refractory just prior to spraying to accelerate the thickening and setting of the refractory castable in order to prevent slumping and sagging of the refractory when it is applied to a vertical surface. It is known that many accelerators have a negative effect on the properties of certain refractories that are typically used for high temperature, highly corrosive environments. For example, sodium silicate is a well-known accelerator used in wet-mix and dry-mix refractory shotcreting. However, even small additions of alkalis, such as sodium, can reduce high temperature strength and refractoriness in high alumina, low cement refractories. It is, therefore, important that an accelerant (i.e., the "accelerator") used with a particular refractory castable be able to provide the thickening and setting characteristics to the particular refractory castable without adversely affecting the properties of the castable by introducing deleterious material into the castable.
The present invention provides a wet-mix shotcreting process for applying a cement-free refractory material of the type described above for use in high temperature applications.